Important properties of diesel fuels include ignition quality, oxidation stability, Ramsbottom carbon and sulfur content. Particularly with respect to ignition quality, cetane number is a limiting specification for diesel fuels. In order to be suitable for automotive use, No. 1 diesel fuel is generally made from virgin stocks having cetane numbers of about 45. Railroad diesel fuels are similar to automotive diesel fuels but can have somewhat lower cetane numbers of about 40.
Many uncracked or virgin paraffinic stocks such as straight run atmospheric gas oil have good compression ignition properties, i.e., a cetane number of about 45 or higher. In contrast, thermally or catalytically cracked stocks, such as cycle oils, have unsatisfactory ignition properties, i.e., cetane numbers below about 35.
In the past, in most countries of the world, sufficient quantities of diesel fuel were obtained as a stable, virgin product from crude oil distillation. However, higher crude prices and poorer quality crude oils have increasingly become an economic reality in refining processes. This has significantly changed the properties of distillate fuels and diesel fuels, especially in the United States. As heavier crudes are being used, bottom products are no longer in demand, and streams from various heavy oil cracking processes have increasingly been used as supplemental blending components for middle distillate fuels. Cracked products generally have poorer qualities as fuels (unless hydrocracked) than straight-run products of equivalent boiling range. With respect to diesel fuels, blending with cracked products has resulted in declining cetane numbers, increasing aromaticity and stability problems in the distillate pool.
The changes discussed above have resulted in a steady decline of cetane number over the past decade. These factors have also led to a loss of distillate fuel stability, which in turn has created problems with diesel fuel handling and performance characteristics. Instability of middle distillates is a result of complex reactions which are not completely understood, but is believed to be the result of three separate reactions: (1) acid-base reactions in which an organic acid and basic nitrogen react to form a sediment (acid-base salt); (2) oxidative gum reactions in which alkenes and oxygen react to form gum and (3) esterification reactions, in which aromatic hydrocarbons, heterocyclic nitrogens and benzothiols combined in a multistep process to form sediment.
Higher levels of unsaturates have resulted from increased use of fluid catalytic cracking units, as well as from blending of streams from thermal processes to meet market demands. The shift to heavier feedstocks and to higher severity operations is significant since, for example, a major change in FCC use could increase the availability of light cycle oil, which is a poor diesel fuel feedstock. The recent emphasis on bottom-of-the-barrel conversion is also expected to increase both nitrogen and sulfur compounds, as well as to produce additional distillate products not well suited for diesel fuel blending.
With an increase in the demand for middle distillates, diesel fuel quality is expected to erode further due to poorer quality of crude oils, a lower demand for bottom products and the increasing use of heavy oil cracking processes.
Recently, treatment to improve distillate quality and stability has been concentrated in three areas: hydro-treating, caustic scrubbing and chemical additives. Although hydro-treating is effective in desulfurization and in improving stability, it is a costly method of improving cetane and stability, requiring a high capital investment, use of hydrogen which is expensive and a high utilities cost relative to other treatment methods.
In refining petroleum distillates the removal of sulfur-containing compounds is also often required to meet product specifications. In the past various methods have been used to remove unwanted sulfur compounds, both by chemical treatment and by hydrodesulfurization. With increasing reliance on high-sulfur crude oil feedstocks, and the desire to divert hydrogen for other uses in the refining process than diesel hydrodesulfurization, chemical desulfurization methods are of increased interest.
The prior approaches involving high temperature, high pressure hydrodesulfurization to reduce the sulfur content of hydrocarbonaceous oils involve a number of major disadvantages. As indicated above, the high temperature, high pressure requirements make these processes quite expensive. The hydrogen required in the processes is expensive and requires water for its production. Further processing of the byproducts produced, such as hydrogen sulfide, which is highly toxic, and ammonia also contribute to the expense of the hydrodesulfurization process. Additionally, the catalyst used is often poisoned by materials contained in the hydrocarbonaceous oil, contributing to a further expense in the process. All of these factors result in economic disadvantage for the known processes.
Strong caustic scrubbing is often employed to remove sediment precursors such as benzenethiol, mercaptan sulfur, H.sub.2 S, acids and phenols from middle distillates. Although caustic scrubbing is often effective, it cannot produce a stable product in all cases, and cannot, for example, remove pyrrolic nitrogen impurities. The disadvantages of caustic treating include cost of maintaining caustic strength, disposal of spent caustic and loss of product by extraction.
Many types of chemical additives are currently used to improve middle distillate fuel quality, alone or in combination with other treatment techniques. Stabilizers generally provide basicity without initially entering into an organic acid-base reaction to form a salt. Antioxidants perform the same function with thermally derived distillates as they do for gasolines. Unsaturates provide free radical precursors that can enter into any of several sediment forming reactions, but these reactions are interrupted by the presence of an antioxidant. Once sediment starts to form, however, stabilizers are less effective and dispersant type additives are used, which cause disassociation of agglomerated sediment particles as well as preventing agglomeration.
Because of the current economic requirement of cutting deeper into the barrel, and the desirability of blending uncracked with catalytically cracked stocks to produce diesel fuels, alternative methods of upgrading diesel fuel to meet the above specifications are now particularly important.
In the petroleum industry, solvent extractions have often been used to remove sulfur and/or nitrogen compounds from petroleum distillates and synfuels, the extract oil and solvent then being separated by distillation. In general, however, solvent extraction of petroleum products to remove sulfur involves a large loss of oil yield and high solvent-to-oil ratio, and provides only limited sulfur removal.
A method of increasing cetane number has long been sought in the art, and it is generally known that the cetane characteristics of a fuel composition containing both aromatic and paraffinic constituents can be improved by removing the aromatic component to increase the concentration of paraffins, e.g., by solvent extraction. However, because aromatics are present in large concentrations, this approach results in uneconomic yield losses when significant improvements in cetane and stability are to be achieved. Further, because of the need to remove large amounts of aromatics and olefins, uneconomically high solvent-to-oil ratios are necessary to provide the requisite solvation capacity. Thus, extraction is not used commercially for these purposes.
It has also long been known that the cetane number of diesel fuels can be improved either by adding a nitrogen-containing fuel additive, or by oxidation with a nitrogenous oxidizing agent. Fuel oils in the diesel range having the proper physical characteristics such as pour point, cloud point, viscosity and volatility can be obtained by nitrogenating the diesel fraction in order to increase the cetane number. However, it is well known that the nitrogenation of such fuel oils tends to increase the Ramsbottom carbon content and to decrease the stability of the oils with formation of an insoluble sediment, which produces a haze and eventually a deposit while the fuel oils are in storage. While many attempts to eliminate the disadvantage of poor stability characteristics have been made and solvent extraction, including caustic scrubbing, has been applied for stability improvement, conventional solvent extraction has proven insufficient to provide sufficient stability in the case of nitrogen-treated fuels at high yields, with sulfur removal, and without cetane loss.
The invention described and claimed herein is directed to a process for upgrading a diesel oil by increasing the cetane rating and reducing Ramsbottom carbon and instability-causing compounds using a nitrogenation/extraction/separation approach in contrast to the generally used catalytic hydrogenation. caustic scrubbing and chemical additive approaches conventionally practiced.
Although various processes for treating petroleum fractions by oxidation or extraction are known, such methods have generally not been satisfactory for upgrading a substandard diesel oil fuel stock by increasing cetane number and improving stability and Ramsbottom carbon of the resulting fuel, with simultaneous sulfur removal if necessary.
Selective solvent extraction to remove aromatic components of petroleum distillates is well known.
U.S. Pat. No. 3,317,423 discloses preparation of a carbon black feedstock by aromatic extraction of a heavy (500.degree. F.+) hydrocarbon using a dual solvent of furfural and a paraffinic hydrocarbon. Preparation of an aromatic carbon black feedstock in a two-stage solvent extraction process using furfural, phenol, liquid sulfur dioxide or glycol ethers is disclosed in U.S. Pat. No. 3,349,028, in which Ramsbottom carbon is also extracted. U.S. Pat. No. 3,415,743 discloses the extraction of heavy aromatics and heavy aliphatics from cycle oil in a two-stage process using dimethyl formamide (5 to 18% water) and xylene in the first stage.
U.S. Pat. No. 3,546,108 discloses a furfural/dimethyl formamide/water mixed solvent used for the extraction of aromatics from gas oils and U.S. Pat. No. 2,137,206 also relates to a method for dewaxing oils using furfural, alone or in combination with auxiliary solvents, such as benzol, benzol and toluol or light petroleum hydrocarbons.
A process for separating petroleum into paraffinic and naphthenic fractions using a mixed solvent including an alkyl-substituted formamide and an alcohol such as methanol or ethylene glycol is disclosed in U.S. Pat. No. 2,183,852. Refining of lubricating oil stock to produce high viscosity index lubricating oil by solvent extraction is disclosed in U.S. Pat. No. 2,067,137, in which acetyl mono- and di- methyl and ethyl amines and corresponding compounds derived from formamide are used as a primary solvent, optionally in combination with a modifying solvent such as benzol, naphtha, propane or butane.
In U.S. Pat. No. 3,169,998 the selective separation of aromatic hydrocarbons from olefinic hydrocarbons, and the extraction of olefinic hydrocarbons from mixtures of olefinic and saturated hydrocarbons is disclosed using liquid gamma-butyrolactone as a solvent. Auxiliary solvents can optionally be used, including sulfur dioxide, sulfolanes, nitriles, ethers, certain glycols, tetrahydrofuran, halogenated hydrocarbon solvents, dimethyl formamide, ketones and aldehydes including furfural. This patent however contains no teaching regarding the importance of prenitrogenation, or of the improvement in cetane and stability which is possible by a combined nitrogenation and cosolvent extraction of diesel oil. Amine sulfonate solvents for extraction of aromatic feedstocks are disclosed in U.S. Pat. No. 2,522,618.
U.S. Pat. No. 3,539,504 describes production of a middle distillate fuel such as kerosene having improved burning and color characteristics by a temperature graduated furfural extraction to remove aromatics and olefins. Auxiliary solvents can optionally be used, including water, sulfolanes, nitriles, ethers, glycols, tetrahydrafuran, halogenated hydrocarbon solvents, dimethyl formamide, ketones, crotonaldehyde, butyrolactone and butyrolactam. There is no disclosure or appreciation of a prenitrogenation step or of its importance in obtaining a diesel fuel having improved cetane and stability when combined with an extraction step using cosolvents.
These patents relating to solvent extraction or solvent/cosolvent extraction all fail to appreciate the importance of nitrogen treatment prior to solvent extraction, and the surprising yield enhancement obtained thereby, or the control of other important product properties, such as stability and Ramsbottom carbon, obtained by the combined use of nitrogen treatment and extraction with selected solvents in the present invention.
Processes for treating petroleum stock by oxidation followed by solvent extraction have been described for various purposes. For example, oxidation/extraction processes of hydrocarbonaceous oils to produce sulfoxides and sulfones are disclosed in U.S. Pat. No. 2,825,744, British Pat. No. 442,524, U.S. Pat. No. 2,702,824, and U.S. Pat. No. 2,925,442.
Further, U.S. Pat. Nos. 3,847,800 and 3,919,402 describe the use of nitrogen oxides followed by extraction by methanol to remove both sulfur and nitrogen compounds from petroleum stocks.
U.S. Pat. No. 4,485,007 discloses a process for purifying hydrocarbonaceous oils containing both heteroatom sulfur and heteroatom nitrogen compound impurities, such as shale oils, by first reacting the hydrocarbonaceous oil with an oxidizing gas containing nitrogen oxides while limiting the molar ratio of the nitrogen oxide to the total sulfur heteroatom content and nitrogen heteroatom content and limiting the conversion of sulfur heteroatom content into gaseous sulfur oxides to about 60% or less on a weight basis, followed by extracting the oxidized oil in one step with an amine selected from the group consisting of ethylene diamine, monoethanolamine, diethanolamine and mixtures thereof, and a second extracting step using formic acid as an extracting solvent. It is disclosed that the amine extracting solvent acts to remove sulfur compound impurities and the formic acid extracting solvent acts to remove nitrogen impurities.
A process for producing a fuel composition by oxidizing a hydrocarbon oil with aqueous nitric acid, followed by extraction with acetone, methyl ethyl ketone, cyclohexanone, methanol, ethanol, normal propanol, isopropanol, ethyl acetate, tetrahydrofuran, dioxane, or a combination of an alcohol and a ketone, an alcohol and water, a ketone and water or a combination of alcohols is disclosed in U.S. Pat. No. 4,280,818.
Although the methods described above have met with some success in desulfurizing petroleum fuels, the known approaches toward oxidation to remove a portion of the original sulfur content as gaseous sulfur oxides, and to convert a portion of the original sulfur content into sulfoxides and/or sulfones followed by extraction with appropriate solvents to achieve a desired low sulfur raffinate have not completely eliminated problems.
Similarly, direct extraction of hydrocarbonaceous oils with selected solvents to remove sulfur and nitrogen impurities to produce a raffinate which is low in sulfur content results in uneconomically low yields of the desired raffinate, along with reductions in the sulfur content of the hydrocarbonaceous oil. The methods described above basically have the disadvantages that (a) the solvents selected are suitable only for specific select oils; (b) the solvents result in poor extraction yields or they do not provide sufficient phase separation to make solvent extraction possible; (c) unacceptably high solvent-to-oil ratios are required, decreasing oil yield and making the processes uneconomical; (d) they require expensive catalyst or extremely severe oxidizing conditions to provide sufficient sulfur removal; or (e) oxidation desulfurization methods involving nitrogenous oxidizing agents often result in increased gum and sedimentation, and reduce the stability of the fuels produced.
For these reasons, the present technology for sulfur removal involving oxidation and subsequent extraction of hydrocarbonaceous oils require improvement.
Similarly, conventional methods of improving diesel cetane number by treatment with nitrogenous oxidizing agents are inadequate to meet other product specifications. Particularly, diesel fuels produced by nitrogenous oxidation and solvent extraction can in some cases meet sulfur and cetane requirements for fuels, but are unsatisfactory with respect to the important specifications of stability and Ramsbottom carbon content. Processes employing sulfuric acid in conjunction with nitrogenous oxidizing agents are ineffective to retain a high cetane rating. Distillative methods are commercially unfeasible due to the presence of substantial carbonaceous deposits in the still, and when thermal treating is applied to diesel fuel to reduce the sulfur content of the residue, this process also produces substantial carbonaceous deposits in the thermal treating still.
Apart from the failure of conventional oxidative cetane enhancement methods to provide diesel fuels of sufficient stability and Ramsbottom carbon content, these methods, like the oxidative desulfurization methods, employ solvents which result in poor yields, requiring unacceptably high solvent-to-oil ratios. Alternatively, the solvents used in some prior methods reduce or entirely eliminate the advantage of cetane enhancement obtained by oxidation.
Particularly, because of the variety of sulfur-containing compounds and instability-causing compounds present in petroleum hydrocarbon feedstocks, and because of the selectivity of solvents for particular sulfur-containing compounds, nitrogen-containing compounds, aromatic compounds and olefinic compounds, previous attempts to upgrade middle distillate fuels by oxidation, solvent extraction or a combination of the two have concentrated on at most one or two product characteristics, and have generally required sacrificing product yield and stability in order to achieve products of acceptable sulfur content or ignition properties.
Although many diesel fuels having low cetane ratings and high sulfur content meet stability and Ramsbottom carbon specifications, if these fuels are oxidized to improve cetane rating or reduce sulfur, Ramsbottom carbon and stability become unacceptable.
Because of these significant disadvantages, conventional oxidation/extraction methods for upgrading middle distillates have largely been supplanted by hydrotreatment, or by chemical additive treatments for improving stability and cetane.
Copending U.S. patent application Ser. No. 832,612, filed Feb. 24, 1986, relates to a method of improving diesel cetane and desulfurization, while retaining acceptable stability and Ramsbottom carbon content, by first contacting a diesel oil with a nitrogenous treating agent and then extracting the treated oil with a selected polar solvent, including furfural, butyrolactone, dimethyl formamide, dimethyl acetamide, methyl carbitol, tetrahydrofurfuryl alcohol, aniline, dimethyl sulfoxide, sulfolane, ethylene chlorohydrin and acetic anhydride. While effective in upgrading a diesel oil and increasing cetane, while retaining acceptable stability and Ramsbottom carbon content, this process is still not completely effective in eliminating the undesirable deterioration in stability caused by nitrogenation.